It may sound odd as a "weapon", but I'd say Self Control/Discipline. If a person disagrees, or otherwise gets mad at me, it often ends with them being even more pissed off (to my amusement) because I completely ignores their lashing out at me, remaining calm.

I have a pretty good control of my verbal and non-verbal communication, and I can adjust what parts of my personality I show. This makes me able to take control of many situations with very little effort.